Some of these buzz like crazy and there isn't much you can do, especially the cheap Chinese ones like Hi-Stop.
What happens with these is that the inside of the hub shell gets some grooves imprinted in it from the expander. Probably during the assembly or testing process. You can sand the inside of the hub shell with emery cloth and repack with a thicker grease which will quiet it down some. The noise should go away after a while with use.